With its diverse resident commensal microbiota, the human microbiome has received substantial attention for the critical roles it plays in health and disease.
An initial line of defense against foreign invaders, the skin is home to a diverse population of microbes. These microbes include resident commensals, transients and pathogens. In order to survive in the challenging environment of the skin, microbes often exhibit a biofilm phenotype, which gives competitive advantages for survival and growth. Many skin pathogens can be found living on the skin as commensals; microbial dysbiosis, host genetic variation, and immune status may drive the transition from commensal to pathogen.
Atopic dermatitis (AD) is a multifactorial chronic inflammatory skin disease involving genetic factors, such as filaggrin deficiency (1), and environmental triggers, including Staphylococcus aureus, and is often the initial presentation of the “allergic march.” Worldwide, AD affects approximately 20% of children and 5% of adults, typically presenting clinically as chronically dry, pruritic eczematous dermatitis with episodic acute flares. AD is frequently associated with asthma and allergic rhinitis. Quality of life for individuals affected by AD is significantly disrupted, and there is a heavy economic burden associated with the disease, with estimates of annual national direct costs ranging from 364M USD to 3.8B USD (4, 5). AD represents a large, unmet need for more effective therapeutics.
Commensal bacteria like S. epidermidis have several antibacterial mechanisms to ward-off pathogenic bacteria. Several studies have shown serine proteins from commensal S. epidermidis prevent the growth of pathogenic strains of S. aureus (40, 41). A novel lipopeptide from commensal Staphylococcus epidermidis increases HβD2 and HβD3 via TLR2/CD36-MAPK, thus enhancing antimicrobial defense against pathogenic infections (42, 43). Other studies have shown that S. epidermidis in the human skin microbiome produces secondary fermentation metabolites to inhibit the growth of additional pathogenic bacterial strains (44). Further, enhancing commensals such as S. epidermidis might have other beneficial roles, too. For example, skin S. epidermidis have an autonomous role in controlling the local inflammatory milieu and tuning resident T lymphocyte function, thereby rendering protective immunity to a cutaneous pathogen (45).
Skin dysbiosis has been linked with common skin conditions, including AD, acne and rosacea (21). Skin microbiome sequencing during acute AD flares has correlated increased levels of the pathogen S. aureus and the commensal S. epidermidis during flares, with subsequent decrease during application of standard medical treatments such as topical steroids and antibiotics (22).
Skin dysbiosis can initiate key biochemical and immune triggers, and studies have found an association between high amounts of staphylococcal bacteria and clinical worsening of AD lesions; for instance, patients harboring MRSA had greater total body clinical dermatitis scores (15, 29-31).
Lipoteichoic acid exerts immunological effects mainly through TLR 2 (32-34) that could implicate it in the worsening of atopic dermatitis (31, 35). Studies showed that in a pediatric population of AD patients, temporal shifts in the skin microbiota occur over three disease stages: baseline, flare, and post flare as compared to healthy controls. In particular, lesional skin bacterial diversity decreased during the flare stage, parallel with increased relative abundance of S. aureus, but increased during the post flare status, indicative of a link between disease severity and microbial diversity (21, 22).
Filaggrin deficiency, Staphylococcus aureus colonization, defective innate immunity and skin microbial dysbiosis are the major underlying factors in the progression of the disease. Up to 90% of individuals with AD are colonized with S. aureus; prevalence of MRSA in AD ranges from 10-30.8% (15).
The role of the skin barrier in the pathogenesis of AD is now clear (18-20). In this context, the strongest genetic association with AD so far has been demonstrated for loss-of-function mutations in the filaggrin (FLG) gene, which encodes the important barrier protein (pro-filaggrin) (8, 9). Filaggrin plays several roles in the pathophysiology of AD which explains why lower expression of a single component of the epidermal differentiation complex might have such a great influence on the whole function of the skin barrier. Skin barrier function is a major determinant of the equilibrium of skin commensal flora. The skin barrier protein FLG ensures that pathogenic strains of bacteria are not penetrating into deeper layers. In a recent study filaggrin knockout resulted in significantly increased epidermal S. aureus colonization, as well as in an up-regulation of S. aureus-induced IL-8 expression (16).
However, the FLG mutation is absent in most AD individuals; secondary filaggrin deficiency is common in AD (6). This has been postulated to be due to various environmental factors, such as the Th2/Th22 cytokine milieu common in AD inflammation, bacterial exotoxins, AD skin dysbiosis associated with cutaneous S. aureus and methicillin-resistant S. aureus (MRSA), mechanical damage associated with scratching, skin dehydration and topical irritants (9-14).
Lower filaggrin levels in AD predispose to S. aureus colonization (11, 12) and lower levels of antimicrobial peptides are associated with defective innate immunity and increased extracellular adhesins for S. aureus (16, 17).
Controlling pathogenic biofilm and its metabolic products on skin is a major component in restoring the symbiosis of commensal skin flora and skin health. Current therapeutics, however, focus mainly on symptom-control rather than modulating the overall microbiome to inhibit the growth and reduce the adhesion and attachment of pathogenic biofilm activities, while promoting the growth of commensal biofilm on human skin.